(1) Field of the Invention
The present invention relates to a wide-area acousto-optic hydrophone used in acoustic arrays for sensing underwater acoustic signals and more particularly to an acousto-optic hydrophone wherein a signal laser beam and a reference laser beam are passed directly through adjacent filled with optically transparent material in such a fashion that each beam undergoes sequential reflections off a series of fixed, rigid mirrors arranged within each chamber in a pattern which folds each beam across a wide area within the bulk material, at the time the bulk material of the signal beam chamber is exposed, through a flexible membrane, to dynamic acoustic pressure variations from the incident signal while the reference beam chamber bulk material experiences only static environmental factors, e.g., temperature and pressure, but not the dynamic ones.
(2) Description of the Prior Art
Conventional sound sensing devices employ magnetostrictive or piezoelectric materials that, when subjected to variations in pressure from impinging acoustic pressure waves, generate electrical signals which are then processed using electronic devices. The use of such hydrophones in acoustic arrays however requires electronic circuitry to each of the hydrophones distributed along the array to provide power and telemetry for operation. Consequently, reductions in the size and weight of acoustic arrays using piezoelectric hydrophones has now reached a practical minimum. In addition, the operational reliability of an acoustic array depends upon the reliability of the electronic devices used to construct that array. What is desirable are hydrophones which do not require any associated electronics thus eliminating the need for electric power to drive such electronic devices, thereby improving the operation of the acoustic arrays.
Acousto-optic transduction is an emerging technology for sensing acoustic pressure by means of light beams. The advancement of fiber optic technology promises to eliminate the telemetry electronics usually associated with currently used piezoelectric hydrophones. Hydrophones constructed using optical sensing offer potentially siginificant improvements in sensitivity, versatility, directionality and reliability over conventional hydrophones. Furthermore, they offer freedom from electromagnetic interference, a reduction in array size, weight and cost together with broad design flexibility for reducing sensitivity to non-acoustic noise.
Optically operated hydrophones have recently been used for frequency modulation of a laser beam or beams exposed to acoustic pressure variations utilizing the doppler effect. These are described in my U.S. Pat. Nos. 4,115,753 and 4,188,096 and in my co-pending patent application; Ser. No. 103,826 filed Dec. 10, 1979. The above described doppler effect devices function based upon the velocity of the moving reflecting surfaces varying the beam path length proportional to the incident acoustic signals. What is desirable is to have an acousto-optic hydrophone which uses laser light to sense only dynamic pressure variations of the incident acoustic waves by offering a wide sensing area to the acoustic signal. The present invention fills this need by sensing incident acoustic signals based solely on bulk material density changes which directly affect the index of refraction of the bulk material and in turn modulates the signal laser beam in proportion thereto by changing the effective optical path length for the beam exposed to the incident acoustic signal.
A wide sensing area is less sensitive to non-acoustic noise such as flow-induced noise while the inherent ability to modify areal beam density permits modifications in hydrophone discrimination. The hydrophone can be operated using a single mode or a multimode optical fiber as a conductor of laser light to and from the hydrophone element.